Evaporation and concentration of liquids, and especially of saline waters, with the vertical tube foam evaporation (VTFE) method have been developed in recent years and patented: U.S. Pat. No. 3,846,254 on Interface Enhancement Applied to Evaporation of Liquids, also patented in Canada, Britain, Australia, South Africa and Israel. An improvement thereof was the subject of U.S. patent application Ser. No. 415,088 filed September 1982, issued as U.S. Pat. No. 4,511,432 on a method of feed distribution for the control of the upflow evaporation process, its stability and thermal performance. The present invention is applied to downflow evaporation as an example of dispersed seeded slurry evaporation (DSSE); DSSE is also applicable to upflow evaporation and other liquid concentration procedures.
The earlier VTFE process provided for the enhancement of the overall heat transfer coefficient (U) or the rate of evaporation by addition of a foaming agent (surfactant) to the liquid to be evaporated and by imposing foamy vapor-liquid-only layer flow over the heat transfer surface to improve evaporation in either the upflow or downflow modes of flow. VTFE enhancement depends on foamy two-phase flow; this enhancement is eliminated by the addition of an anti-foaming agent such as a silicon based foam breaker (Ref: Fong et al, Canadian Society for Chemical Engineering, Paper No. CSChE/CSME-75-Ht-1, 1975, page M7/8, or a commercial silicone suspension. The DSSE process however does not depend on foamy two-phase flow, is not eliminated by an anti-foaming agent addition, and is effective at liquid-solid, submerged interfaces where vapor or foam is remote.
In recent developments by others with brine evaporators used for the disposal of wastewaters, and relying on seeded slurry brine recycle for scale control, they have reported serious scaling even with the addition of the conventional scale inhibitors i.e. polyphosphates and polyacrylates; such scaling required frequent and costly interruptions for descaling. Such descaling has been needed on about a 3 to 6-month cycle, with chemicals or by hydroblasting the scale from evaporator surfaces, and has reduced evaporator productivity rates to about 60% of design capacity.
In the present invention for the concentration of liquids, the dispersant-dispersed seeded slurry evaporation (DSSE) process provides improvements by the addition of a dispersant or a dispersing agent (e.g. a surfactant selected for its dispersing effects) that inhibits scaling and fouling of equipment surfaces including submerged surfaces where vapor and foam are absent. Also, the preferred dispersant has been discovered to have the effect of improving the rate of descaling by causing redispersion and dissolution of such DSSE scale into pure liquid, or into non-saturated liquid. While such scales or deposits are inhibited with DSSE, they may yet eventually form on equipment surfaces but with incorporation of this dispersant therein. Descaling with a solvent is then assisted by the dispersant included in scales.
This DSSE process has been tested for its application to the concentration of industrial wastewater, a saline water or brine. The disposal of power plant cooling tower blowdown by many-fold concentration thereof during evaporation, utilizing waste heat, has thus been tested. It has been found that the DSSE brine concentration process operates with improved thermal efficiency due to scale-free operation for long (extended) periods of time. Also, that this scale-free operation is not negated by the addition of a commercial silicon based foam breaker or a silicone anti-foaming agent. DSSE scale control remained effective for evaporation tube surfaces as well as for submerged equipment surface sites that are free of vapor or foam contact. The DSSE dispersant agent is thereby distinguished from a VTFE foaming agent in its selection, its physical-chemical function, the site and the mode of its operation, and its use or application herein.
The DSSE additives are also distinguished from the conventional polymeric scale inhibitors above, as well as others proposed in the literature, by their basis for selection: DSSE dispersants are included in or incorporated with precipitates and scales and render them readily redispersed or redissolved in non-saturated liquid or in fresh water. Such selected dispersants include members of surfactant series that have a strong hydrophilic (or liquiphilic) group, are strongly polarized or negatively charged, or anionic, and monomeric when in dilute solution. Such selected dispersants may or may not cause foamy flow in a liquid evaporated by DSSE. A typical example of such a selected or preferred dispersant for DSSE is dodecylbenzene sulfonic acid or its salts. More effective DSSE additives of the alkylbenzene sulfonic acid type disclosed herein developed, found or selected later are understood as included under this disclosure, for DSSE uses.